Display Value of a nodes RGB input in a panel
Use the indexed element, eg red is index 0



For array properties can display an individual element in a layout.prop(...) by specifying the index of the array element.  Eg for the red channel of mix color input input.default_value

layout.prop(input, "default_value", index=0)


The color node socket input has a type 'RGBA'

def draw(self, context):
    layout = self.layout
    # node for testing
    node = bpy.data.materials["Material"].node_tree.nodes["Mix"]
    input = node.inputs["Color1"]
    col = layout.column(align=True)
    for i, c in enumerate(input.type): 
        col.prop(
                input, "default_value", 
                text=c.upper(),
                index=i)